This invention relates to a continuously variable transmission, and more particularly to a compact continuously variable transmission which is combined with a fluid coupling to assure a good speed changing characteristic.
In a continuously variable transmission employed popularly, the speed change ratio cannot be set to infinity. Therefore, a continuously variable transmission for an automobile wherein an internal combustion engine is employed as a prime mover requires a coupling upon starting. Specific samples of these couplings include a fluid coupling and a torque converter which employ fluid, and a friction clutch which makes use of mechanical friction.
Meanwhile, a vehicle requires a mechanism for changing-over neutral, forward running and backward running states from each other since not only forward running but also backward running are required. Therefore, a counter gear or a planetary gear apparatus is used to perform the change-over by means of a clutch of the dog type or by means of a hydraulic clutch or a hydraulic brake. However, since gears for forward running and backward running are required, not only the axial length of the continuously variable transmission is increased by them, but also a high cost is required.
Thus, a continuously variable transmission has been also proposed, of a type which includes a torque converter which changes the direction of rotation of a belt to drive the vehicle to run forwardly or backwardly.
The torque converter includes a pump runner connected to an internal combustion engine side, a stator runner whose rotation is inhibited by a brake mechanism, and a turbine runner connected to a pulley side. In the torque converter, rotation of the stator runner is permitted or inhibited to rotate the turbine runner forwardly or backwardly to change the direction of rotation of the belt thereby to cause the vehicle to run forwardly or backwardly.
The structure of the torque converter of the type just described will be described in more detail below with reference to FIG. 5 which shows a torque converter for a continuously variable transmission disclosed in Japanese Patent Laid-Open Application No. Hei 2-159451.
The torque converter shown is generally denoted at 01 in FIG. 5. In the torque converter 01, driving torque of an internal combustion engine drives a pump 03 to rotate via a drive plate 02. A turbine 04 is connected to a shaft 05, which is disconnectably connected to the pump 03 by means of a direct coupled clutch 06. The shaft 05 and a pulley input shaft 08 are disconnectably connected to each other by a neutral clutch 07.
Further, a stator 10 is interposed between a pump runner and a turbine runner via a stator connector member 12 provided on a shaft 11, and a brake 13 is interposed between the other end of the shaft 11 and a casing 09.
Problems of the torque converter described above will be examined here.
(1) Characteristics upon Starting
For a fluid coupling such as a torque converter, a stall matching characteristic of an engine makes a significant factor on the fuel cost, the acceleration, and the feeling upon starting. In addition, the stall matching characteristic of an engine is expressed by the rotational speed of the engine in such a state that the accelerator pedal is operated fully when a turbine connected to the output of the fluid coupling, that is, the vehicle is in a stopping state.
In such a torque converter as shown in FIG. 5, upon starting for forward running, the stator 10 idles at a speed substantially equal to the speed of the pump 03 connected to the engine, and thereby the pump 03 does not disturb a flow of fluid to the turbine 04. However, upon starting for backward running, since the stator 10 is fixed, the pump 03 disturbs a flow of the torque transmitting fluid, and consequently, the flow rate of the torque transmitting fluid is decreased. Consequently, the capacity of the pump is decreased, and the speed of the engine becomes higher than that upon forward running.
Usually, since a torque converter is designed so as to assure a good stall matching characteristic for starting for forward running, the starting characteristic upon backward running is degraded resultantly.
(2) Position of Stator in Torque Converter
If the stator 10 is designed so that it is positioned on the outer peripheries of the pump 03 and the turbine 04 in such a manner as to cover over them, then it has significantly bad effects on the cost and the performance.
In particular, where the structure just described is employed, since the stator 10 is positioned on the outer peripheries of the pump 03 and the turbine 04, if it is intended to obtain a same torque capacity (as that, for example, achieved by a torque converter to which the present invention is applied), then the outer diameter of the torque converter is increased by the stator 10, resulting in the increased cost. Since also the stator 10 itself is naturally positioned on the outer periphery of the torque converter, it is increased in size and hence in cost.
Accordingly, in most automobiles at present, the position of a stator is selected on the inner periphery sides of a pump and a turbine.
Further, the stator connector member 12 for interconnecting the stator runner to the brake 13 must extend across flow paths of the pump 03 and the turbine 04. Since the stator runner is applied with a thrust force in an axial direction by the fluid, the crossing portions of the stator connector member 12 must have a sufficient strength. Further, although each of the crossing portions of the stator connector member 12 has a hole, the formation of the hole disturbs a flow of fluid in the inside of the torque converter and gives rise to a disadvantage that the torque capacity is decreased.